An optical probe for measuring physical and chemical characteristics of a transparent fluid medium is intended to be placed within the flow, and comprises two rigid and spaced-apart fingers having respective longitudinal axes which are parallel. The probe is oriented so that these longitudinal axes of the fingers are also parallel to the velocity vector of the flow. Each finger has an upstream end within the flow, a downstream end, and a side region of optical quality for relaying the light beam between the finger and the medium.
The downstream end of the first of the two fingers is adapted to receive the light beam issuing from the source, and the downstream end of the second of the two fingers is adapted to transmit this light beam towards a detector.
In a known manner, each upstream end of the fingers has an end face which is oblique relative to the longitudinal axis, forming together with a side surface of the same finger, a projecting corner in order to split the flow. Such a shape of the finger ends which are upstream in the flow reduces the disturbances to the flow caused by the probe, thus also reducing changes to the medium which are caused by disturbances to the flow. For example, when the flow is supersonic, the upstream end of each finger causes a shock wave in the flow, which in turn causes a variation in the density of the medium. Having the upstream ends of the fingers shaped as a projecting corner reduces such disturbing effects, so that the characteristics of the medium during flow which are obtained with the probe are more representative of the state of the medium when the probe is not present.
The side regions of optical quality of the two fingers are situated so as to transmit the light beam from one to the other through the medium, in a path between the two fingers which is not orthogonal to the velocity vector of the flow. Thus the Doppler effect which affects the detection of spectroscopic absorption bands of the medium allows obtaining a measurement of the velocity of the flow.
The article entitled “Measurements of Gas Temperature and Velocity in Hypervelocity Flows Using Diode-Laser Sensors” by S. D. Wehe et al., of the American Institute of Aeronautics and Astronautics, Inc., 1998, describes such a probe and its use. In the probe described, each finger is a hollow tube which contains several optical components such as mirrors, prisms, and lenses. The light beam is propagated within the free space inside each tube, and enters or exits through the side windows constituting the finger's regions of optical quality. By construction, these windows are at a distance from the upstream end face of the corresponding finger, in each of the two fingers of the probe. It is important to note that on the finger furthest upstream, the light beam is divided into a measurement beam that is oblique relative to the axis of the fingers in order to be able to measure a Doppler effect in the flow, and a reference beam perpendicular to the axis of the finger to avoid any contribution from this effect. FIG. 1 is a cross-sectional view of such an optical probe, with the following labels:                M: medium in which the probe is placed,        V: flow velocity vector of the medium,        F: light beam,        1: finger supplying the light beam F,        A1: longitudinal axis of finger 1,        11: upstream end face of finger 1,        12: side face of finger 1,        13: optical window of finger 1,        14: internal mirror of finger 1,        15: optical fiber for introducing the light beam F,        16: downstream end of finger 1,        2: finger where the light beam F is recovered;        A2: longitudinal axis of finger 2,        21: upstream end face of finger 2,        22: side face of finger 2,        23: optical window of finger 2,        24: internal mirror of finger 2,        25: light detector,        26: downstream end of finger 2.        
For finger 1 (respectively 2), the end face 11 (resp. 21) together with the side face 12 (resp. 22) forms a corner projecting at an acute angle which enters the flowing medium M and has the sole function of minimizing the deviation of the streamlines around the probe in comparison to the flow when the probe is not present.
A disturbance of the flow and/or of the medium M which is generated at the upstream end of each finger 1 (resp. 2) is carried to its downstream end by the flow itself. It then affects a portion P of the medium M which is increasingly large in planes perpendicular to the longitudinal axis A1 (resp. A2) the further away these planes are from the upstream end face 11 (resp. 21) in the direction of the downstream end 16 (resp. 26). Depending on the velocity of the flow, the disturbed portion P of the medium M can be an area of turbulence created in the flow by each finger, or a cone of a supersonic shock wave issuing from each upstream finger end. The crosshatched areas in FIG. 1 indicate these disturbed portions P of the medium M between the two fingers. Because of the distance between the upstream end 11, 21 and the optical window 13, 23 for each finger 1, 2, a significant proportion of the length of the path of the beam F between the two fingers 1 and 2 lies within these disturbed portions P of the medium M. This results in distortion of the physical and chemical characteristics obtained with the probe, in comparison to the values of these same characteristics when no probe is present.
In addition, the structure of such an optical probe has the following disadvantages, particularly due to there being multiple components:                its high price;        the difficulty of its assembly, particularly in positioning and orienting each optical component so that the light beam F follows the correct path from the fiber 15 to the mirror 14, then to the mirror 24 through the windows 13 and 23, and finally to the detector 25;        the probe is sensitive to impacts and vibrations which could cause misalignment of some of these optical components. Errors can then result when measuring the characteristics of the flow and the medium M;        the structure of a probe with multiple components does not allow reducing its dimensions, which are on the order of 10 cm (centimeters) for the length L1 of finger 1 and 5 cm for the distance De between the longitudinal axes A1 and A2. Due to this, the spatial resolution of measurements which can be obtained with the probe is limited. In addition, the disturbance created by the probe in the flow cannot be reduced for this reason; and        lastly, the propagation of the light beam F inside each hollow finger tube can be disturbed by accidental entry of the medium M into these tubes. Such entry of the medium M into the tubes of the fingers also alters the results obtained with the optical probe for the characteristics of the flow and the medium M.        
The article by the same authors, S. D. Wehe et al., entitled “Diode-Laser Sensor for Velocity Measurements in Hypervelocity Flows”, AIAA Journal, Vol. 37, No. 8, concerns the same type of probe.
Lastly, document DE 10 2008 050109, which also corresponds to US 2010/0027015, discloses an optical sensor in which the two fingers (“light guides”) are of the same length, so that the light beam follows a path between these two fingers which is perpendicular to the fingers themselves. Such a sensor is suitable for measurements in a transparent fluid medium which is static, but it does not allow conducting measurements based on a Doppler effect generated by a flow of the medium parallel to the fingers. The sole function of the corners projecting at an acute angle at the end of the fingers is to support the mirrors.
In addition, the probe in documents DE 10 2008 050109 and US 2010/0027015 has oblique mirrors which are arranged at the ends of the fingers and which are constructed of thin layers, specifically layers of metal. Such layers are sensitive to the corrosion and/or ablation that may be caused by certain fluid mediums in which the probes may be used. The operation of the probe then progressively deteriorates.